German Foreign Minister Guido Westerwelle is currently visiting Mexico.

Before the Thai minister's arrival however, he said the government could not comment on the affair "because we respect the independence of the judicial authorities" handling the case.

On Tuesday, German officials sealed a Boeing 737 often flown by the heir to the Thai throne Maha Vajiralongkorn, and banned it from taking off, Munich airport said. The move came because of a long-running business dispute.

"Legally, this is a huge mistake," Kasit told reporters in Bangkok before leaving for Europe. In Thailand the royal family is a revered institution.

In talks in Berlin, Pieper regretted "the inconvenience caused to the crown prince by this seizure."

But she reiterated Westerwelle's position that the matter was in the hands of the German courts.

In a statement the two ministers agreed that the matter should not harm bilateral relations.

Werner Schneider, insolvency administrator for the Walter Bau firm, said the seizure followed repeated refusals by the Thai government to pay money it says it is owed.

"We have been trying for years ... to have our justified demands for more than 30 million euros ($42 million) met, and this drastic measure is basically the last resort," Schneider's firm said in a statement.

"The Thai government keeps playing for time and has not reacted to Schneider's demands. Even the involvement of the relevant departments of the German government proved fruitless."

The dispute goes back more than 20 years to the involvement of DYWIDAG, which merged with Walter Bau in 2001, in building a motorway link between Bangkok and Don Muang airport.

After "numerous breaches of contract by the Thai government", Walter Bau, by then insolvent, in 2007 claimed for damages. A court ruled in his favour in 2009, Schneider said.

Kasit, speaking in Bangkok said Thailand wanted "the German court to act on our request revoke the seizure immediately."

He added that the plane belonged to the crown prince, not the government.

"If this takes too long it might affect the feelings of Thai people towards German people and the country because this is related to the monarchy," he warned.